Problems associated with the generation and discharge of electrostatic charge during the manufacture and use of photographic film and paper have been recognized for many years by the photographic industry. The accumulation of charge on film or paper surfaces can cause difficulties in support conveyance as well as lead to the attraction of dust, which can produce fog, desensitization, repellency spots during emulsion coating, and other physical defects. The discharge of accumulated static charge during or after the application of the sensitized emulsion layer(s) can produce irregular fog patterns or static marks in the emulsion. The severity of the static problems has been exacerbated greatly by increases in sensitivity of new emulsions, increases in coating machine speeds, and increases in post-coating drying efficiency. The charge generated during the coating process results primarily from the tendency of webs of high dielectric constant polymeric film base to undergo triboelectric charging during winding and unwinding operations, during conveyance through the coating machines, and during post-coating operations such as slitting, perforating, and spooling. Static charge can also be generated during the use of the finished photographic product. The repeated winding and unwinding of film in and out of a film cassette or cartridge can result in the generation of electrostatic charge, especially in a low relative humidity environment. The accumulation of charge on the film surface results in the attraction and adhesion of dust to the film and can even produce static marking. Similarly, high-speed automated film processing equipment can generate static that produces marking. Sheet films are especially subject to static charging during use in automated high-speed film cassette loaders (e.g., x-ray films, graphic arts films, microfiche, etc.).
An electrically-conductive layer can be incorporated into an imaging element in various ways to dissipate accumulated static charge, for example, as a subbing layer, an intermediate layer or interlayer, and especially as an outermost layer either overlying the imaging layer or as a backing layer on the opposite side of the support from the imaging layer(s). A wide variety of conductive antistatic agents can be used in antistatic layers to produce a broad range of surface electrical conductivities. Many of the traditional antistatic layers used in imaging elements employ electrically-conductive materials which exhibit predominantly ionic conductivity, for example, simple inorganic salts, alkali metal salts of surfactants, alkali metal ion-stabilized colloidal metal oxide sols, ionic conductive polymers or polymeric electrolytes containing alkali metal salts and the like. The electrical conductivities of such ionic conductors are typically strongly dependent on the temperature and relative humidity of their environment. At low relative humidities and temperatures, the diffusional mobilities of the charge-carrying ions are greatly reduced and the bulk electrical conductivity is substantially decreased. At high relative humidities, an unprotected ionic antistatic backing layer containing such an ionic conductor can absorb water, swell, and soften. Especially in the case of photographic roll films, this can result in the adhesion (viz., ferrotyping) and even physical transfer of portions of a backing layer to a surface layer on the emulsion side of the film (viz., blocking).
Antistatic layers containing electronic conductors such as conjugated conductive polymers, conductive carbon particles or fibers, metallic particles or fibers, crystalline semiconductor particles, amorphous semiconductive fibrils, and continuous semiconductive thin films can be used more effectively than ionic conductors to dissipate static charge since their electrical conductivity is independent of relative humidity and only slightly influenced by ambient temperature. Of the various types of electronic conductors, electrically-conductive metal-containing particles, such as semiconductive metal oxides, when dispersed with suitable polymeric film-forming binders, are particularly effective for use in transparent conductive layers. Binary metal oxides doped with appropriate donor heteroatoms or containing oxygen deficiencies have been disclosed in prior art to be useful in antistatic layers for photographic elements, for example: U.S. Pat. Nos. 4,275,103; 4,416,963; 4,495,276; 4,394,441; 4,418,141; 4,431,764; 4,495,276; 4,571,361; 4,999,276; 5,122,445; 5,294,525; 5,382,494; 5,459,021; and others. Suitable claimed conductive metal oxides include: zinc oxide, titania, tin oxide, alumina, indium oxide, silica, magnesia, zirconia, barium oxide, molybdenum trioxide, tungsten trioxide, and vanadium pentoxide. Preferred doped conductive metal oxide granular particles include Sb-doped tin oxide, Al-doped zinc oxide, and Nb-doped titanic Additional preferred conductive ternary metal oxides disclosed in U.S. Pat. No. 5,368,995 include zinc antimonate and indium antimonate. Other suitable conductive metal-containing granular particles including metal borides, carbides, nitrides, and suicides have been disclosed in Japanese Kokai No. JP 04-055,492.
Antistatic backing or subbing layers containing colloidal amorphous vanadium pentoxide, especially silver-doped vanadium pentoxide, are described in U.S. Pat. Nos. 4,203,769 and 5,439,785. Colloidal vanadium pentoxide is composed of highly entangled microscopic fibrils or ribbons 0.005-0.01 .mu.m wide, about 0.001 .mu.m thick, and 0.1-1 .mu.m in length. However, colloidal vanadium pentoxide is soluble at the high pH typical of developer solutions for photographic processing and must be protected by a nonpermeable, barrier layer as taught in U.S. Pat. Nos. 5,006,451; 5,221,598; 5,284,714; and 5,366,855, for example. Alternatively, a film-forming sulfopolyester latex or polyesterionomer binder can be combined with the colloidal vanadium pentoxide in the conductive layer to minimize degradation during processing as taught in U.S. Pat. Nos. 5,360,706; 5,380,584; 5,427,835; 5,576,163; and others. While the use of a polyesterionomer binder provides improved coating solution stability and enhanced interlayer adhesion, a hydrophobic overcoat still must be provided in order to ensure the degree of process-surviving capabilities desirable for photographic imaging elements. The need to overcoat the antistatic layer with such a hydrophobic barrier layer has several potential disadvantages including increased manufacturing cost and complexity; inability to use the antistatic layer as the outermost layer; and limited ability to overcoat the antistatic layer directly with a hydrophilic, water swellable layer such as a curl control layer or pelloid. Thus, it is desirable to avoid the use of a hydrophobic barrier layer overlying an antistatic layer in a photographic element.
The use of non-conductive "auxilliary" fine particles such as binary metal oxides (e.g., ZnO, TiO.sub.2, SiO.sub.2, Al.sub.2 O.sub.3, MgO, BaO, WO.sub.3, MoO.sub.3, ZrO.sub.2, P.sub.2 O.sub.5), kaolin, talc, mica, alkaline earth sulfates (e.g., BaSO.sub.4, SrSO.sub.4, CaSO.sub.4, MgSO.sub.4) or alkaline earth carbonates (e.g., CaCO.sub.3, MgCO.sub.3) as grinding aids in the preparation of electrically-conductive layers containing conductive metal oxide particles for use in photographic elements has been disclosed in U.S. Pat. Nos. 4,416,963; 4,495,276; 5,028,580, and 5,582,959. It was claimed in the '580 Patent that "fine grains" of a crystalline non-conductive metal oxide which do not contribute directly to improving conductivity can be added to a backing layer for a thermal recording imaging element. It was further disclosed in the '580 patent that it was particularly advantageous to remove a greater part of any such auxilliary particles by physical (e.g., filtration, centrifugation, etc.) or chemical (e.g., dissolution) treatments after preparing dispersions of the conductive metal oxide particles and before preparing coated layers.
Colloidal silica in the form of an aqueous sol consisting of silica particles with a high specific surface area can be used in combination with a soluble alkylaryl polyether sulphonate to provide conductive backing layers for photographic paper as disclosed in U.S. Pat. No. 3,525,621. However, such conductive layers exhibit unsuitably low levels of conductivity after photographic processing because of the solubility of the alkylaryl polyether sulphonate in the photographic processing solutions.
The use of colloidal non-conductive metal oxide particles combined with an optional film-forming polymeric binder to prepare conductive layers for photographic elements has been taught widely in prior art. For example, the preparation of conductive layers comprising a continuous gelled network of colloidal metal oxide particles on a photographic film or paper support is taught in European Application Nos. 250,154; 301,827; 531,006; 657,774. Preferred colloidal particles are disclosed to have an average diameter less than about 20 nm. The inclusion of an ambifunctional silane compound as a coupling agent in conductive layers containing colloidal metal oxide particles to improve adhesion to overlying gelatin-containing layers was claimed in U.S. Pat. No. 5,204,219. Preferred colloidal metal oxide particles claimed include silica, titania, and tin oxide, and mixtures thereof. Dispersions of such colloidal metal oxide particles are typically stabilized electrostatically by the presence of alkali metal or ammonium cations. The use of gelatin or a gelatin-compatible protein such as chitosan (i.e., a d-glucosamine) as a film-forming polymeric binder is taught in European Application Nos. 657,774 and 531,006, respectively. Coatings prepared in accordance with European Application No. 531,006 contain colloidal metal oxide particles at a particle to polymeric binder weight ratio of from 75:25 to 92:8 and exhibit surface resistivity values of 8-10 log ohms/square at 40% R.H. which increased to 10-11 log ohms/square after photographic processing. The addition of a water soluble alkali metal orthosilicate or metasilicate to a conductive layer containing a gelled network of colloidal metal oxide particles in order to improve cohesion as well as adhesion of the layer when overcoated with a gelatin-containing layer is taught in U.S. Pat. Nos. 5,236,818 and 5,344,751 and European Application No. 657,774.
The use of colloidal metal oxides in the presence of alkali metal ions, multifunctional silanes, and various ionic conductive polymers such as sodium styrenesulfonate/maleic acid copolymers in antistatic layers for photographic elements is disclosed in European Application No. 618,489. Preferred colloidal metal oxides include titania, silica, and alumina. Antistatic coatings were reported to exhibit surface resistivity values ranging from 6 to 10 log ohms/square. However, no values were reported for the antistatic coatings after photographic processing. The use of colloidal metal oxide particles in combination with various organosilanes in transparent abrasion-resistant protective topcoatings for polymeric sheets or articles is disclosed in U.S. Pat. No. 4,571,365. The use of non-conductive colloidal metal oxides (e.g., alumina, antimony oxide) as well as conductive metal oxides (e.g., antimony-doped tin oxide, tin doped indium oxide, cadmium stannate) in such coatings was claimed. Use of conductive metal oxides was reported to improve static dissipating properties of the protective layers. However, the utility of such protective layers for imaging elements was neither disclosed nor anticipated.
A class of composite, electrically-conductive powders said to be useful for preparing conductive coatings, films, and other articles consisting of an intimate mixture of at least one type of electrically-conductive powder and at least one type of a particulate filler material which is nonconductive has been disclosed in U.S. Pat. No. 5,545,250. Such composite conductive powders preferably contain binary or ternary mixtures of the component powders. Further, the composite conductive powders are said to exhibit dry powder resistivity values which are lower than the weighted average of the dry powder resistivity values for the component powders. Suitable electrically-conductive component powders include crystalline antimony-doped tin oxide particles as well as composite conductive particles consisting of non-conductive core particles such as oxides of titanium, silicon, magnesium, calcium, barium strontium, zinc, tin, nickel or iron; carbonates or sulfates of calcium, barium or strontium; mica, cordierite, anorthite, pyrophyllite, and the like, upon which an amorphous silica coating and a network of conductive crystallites (e.g., antimony-doped tin oxide, silver, gold, copper, nickel, etc.) are deposited sequentially. Specific methods for preparing such composite conductive particles have been described in detail in U.S. Pat. Nos. 5,024,826 and 5,236,737. Suitable non-conductive filler powders include amorphous silica, hollow silica shells, titania, mica, calcium carbonate, as well as the core particles used to prepare the composite conductive particles described hereinabove. The method used to prepare the composite electrically-conductive powders of the '250 Patent is described as essentially a relatively gentle dry blending procedure that is sufficient to provide intimate mixing of the individual component powders without degrading the electroconductive properties of the conductive component powders. Further, conductive coatings prepared using dispersions of composite conductive powders of the type taught in the '250 Patent having a film-forming binder in an aqueous vehicle are said to exhibit less color and higher optical transparency than such coatings containing equivalent amounts of the electrically-conductive component powders. However, the particles comprising the composite conductive powders as well as the conductive and non-conductive component particles taught in the '250 Patent are substantially too large to provide conductive layers with optical transparency and low haze properties suitable for use in photographic or thermally-processable imaging elements.
The use of colloidal, electrically-conductive metal antimonate particles (e.g., zinc antimonate particles) in antistatic layers for imaging elements, especially for silver halide-based photographic elements, is broadly claimed in U.S. Pat. No. 5,368,995. However, dry weight coverages of zinc antimonate in conductive subbing and backing layers sufficient to provide preferred levels of electrical conductivity for antistatic protection of imaging elements, especially in the case of micrographic films and other films, which are sensitized primarily to blue light, produce an undesirable increase in optical density and haze. The requirements for low optical density, low haze, lack of photoactivity, and low manufacturing cost dictate that the conductive layer must be coated using as low a dry weight coverage of metal antimonate as possible. One objective of the present invention is to provide transparent, conductive layers which deliver adequate antistatic protection as well as exhibit lower optical absorption and scattering losses than conductive layers containing metal antimonate of prior art. It is a further objective of the present invention to maintain the required physical properties of the conductive layers such as adhesion to overlying or underlying layers when used as a subbing layer, an interlayer or a protective layer, cohesion of the conductive layer in order to minimize dusting, and scratch and abrasion resistance when the conductive layer is to be used as an outermost protective layer such as a backing or overcoat layer. Because the requirements for a conductive layer to be useful in an imaging element are extremely demanding, the art has long sought to develop improved conductive layers exhibiting a suitable balance of all the necessary chemical, physical, optical, and electrical properties.